Pu-238 is radioactive by alpha-decay (half-life 87.7 y). Each decay releases 5.6 MeV of energy. Pu-238 is sometimes used to power space probes. If all the decay energy can be converted to electric power, what mass of Pu-238 would be required to power a 22 kW space probe for at least 150 y? [Use 238 for the atomic mass of Pu-238]